Mostly people are living longer. When SS first started only 55% of males and 60% of females who made it to age 21 would have survived their working-age years to retire at age 65 in 1940. By 1990 that figure had risen to 72% and 83%, respectively. Life expectancy after retirement also increased, from 12.7 (M) or 14.7 (F) years in 1940 to 15.3 or 19.6 years in 1990. (https://www.ssa.gov/history/lifeexpect.html)